This invention relates to a technique for detecting flaws on surfaces, and, more particularly, to automatically and dynamically correcting for variations in signals from position-to-position as well as drift of various parameters with time.
Before the development of this technique, tubing inspection on nuclear reactor steam generator tubing and the Zircaloy nuclear reactor fuel rods was performed manually. Such visual flaw/defect inspection tasks are dependent upon human operation and interpretation. This method lacks a definitive and accurate reference, lacks consistency, is slow and tedious, and adds significantly to product cost. Where ultra-high reliability is required, however, none of the prior art automatic scanning systems were able to locate defects as effectively as the human inspectors.
A large number of surface scanning systems have been proposed in the past. U.S. Pat. No. 2,975,293 issued to Kruse et al. on Mar. 14, 1961 and U.S. Pat. No. 3,804,534, issued to Clarke on Apr. 16, 1974 illustrate such scanning systems. The use of more than one detector for rough surfaces is taught, for example, in U.S. Pat. No. 3,984,189 issued to Seki et al. Generally, the prior art scanning systems were designed for flat surfaces, but U.S. Pat. No. 3,749,496 issued to Hietanen et al. on July 31, 1973 illustrates the inspection of the inside surface of a cylindrical workpiece. Generally the inspection device is stationary and the surface is moved past it.
Some systems have used electronic logic or memories for defect evaluation including means to prevent indications of multiple flaws when a single flaw is scanned by consecutive scans. See, for example, the following U.S. Pat.: Nos. 3,900,265 issued to Merlen et al. on Aug. 19, 1975; 3,812,373 issued to Hosoe et al. on May 21, 1974; and No. 3,781,117 issued to Laycak et al. on Dec. 25, 1973. Some systems use averaging of the signal from the preceding portion of the scan (passing the scan signal through a low pass filter) to develop a base line signal, and then compare the instantaneous signal to the base line signal in order to partially compensate for the gradual variations in sensitivity throughout a scan. U.S. Pat. Nos. 3,781,531 issued to Baker on Dec. 25, 1973 and 3,920,970 issued to Slaker on Nov. 18, 1975 illustrate this technique. The sensitivity of such circuits is limited as the error threshold must be set away from the base line not only by the normal amount of noise, but also by the amount which the base line shifts during the filtering period.
While the prior art methods have been satisfactory for some applications, higher quality products, such as nuclear reactor fuel tubing and nuclear reactor steam generator tubing have required closer inspection than what was provided by the prior art automatic scanning systems. As a result, slow manual inspection methods have been heretofore used for such products.